Evidence on the monophyly of Astragalus (Fabaceae) and its major subgroupsbased on nuclear ribosomal DNA ITS and chloroplast DNA trnL intron data

Phylogenetic relationships among 115 species representing the legume genus Astragalus and It related genera were inferred from an analysis of nucleoti de sequence variation in the internal transcribed spacers and 5.8S gene of nuclear ribosomal DNA. For a subset of these taxa, the ITS data were supple mented by sequences from the chloroplast trnL, intron. Phylogenies derived from maximum parsimony and neighbor-joining analyses of sequence and insert ion/deletion characters all suggest that the vast majority of Astragalus is monophyletic (with the exception of "outlier" species). All New World Astr agalus species with aneuploid chromosome numbers (n = 11-15) form a monophy letic group ("Neo-Astragalus") which now includes the Mediterranean aneuplo id Astragalus echinatus. Other Old World aneuploid species are not closely related to Neo-Astragalus, but rather are found among Old World euploid (n = 8, 16) groups. Similarly, the relatively few North American species with euploid numbers are not the closest relatives to Neo-Astragalus but are dis persed among divergent Old World groups that include both aneuploid and eup loid species. The historically allied genus Oxytropis is not nested within Astragalus, but forms a separate clade within the larger "Astragalean" clad e. The proposed segregate genera Astracantha (Eurasian) and Orophaca (North American) are clearly nested within Astragalus s, str. South American spec ies of Astragalus are nested within Neo-Astragalus and comprise at least tw o independently derived clades (along with their close North American relat ives), as previously suggested by morphology. Parsimony reconstructions of characters that have been used in the traditional subgeneric taxonomy of th e genus were examined and show high levels of homoplasy. Preliminary estima tes of the absolute rate of species diversification in Astragalus suggest i t may be higher than in some other, often cited, continental or insular ada ptive radiations in angiosperms.